The difference between Factor and Image: How can I combine them?
It is easier to understand if you use a black and white image to combine the elements.
This black and white image has two values: 0 means Black, and 1 means White.

The Factor input determines how two other elements in a node are combined:
Whatever is black will be the color of the Top image socket,
Whatever is white will be the image connected to the Bottom socket.

All other values between 0 and 1 (different tones of gray) in the Factor image, will result in a mix between the two values.